The present invention relates to a fuel cell stack according to the definition of the species in claim 1.
Fuel cell stacks according to the related art include at least one, however, typically a plurality, of individual fuel cells which are stacked either side by side or one upon the other. An individual cell includes two distribution plates for the distribution of the fluids and a membrane electrode unit, also abbreviated referred to as MEA (membrane electrode assembly), situated between them. A MEA includes an anode, a cathode, and a proton-conductive electrolyte membrane situated between them. By using the proton-conductive electrolyte membrane (PEM), proton conveyance from the anode to the cathode is ensured. On the anode side and the cathode side, the distribution plates have gas channels (anode and cathode channels) for the supply and the removal of the fuel-containing anode gas, e.g., hydrogen, and the oxygen-containing cathode gas, e.g., air.
In an electrochemical reaction, water is obtained as a product at the cathode. Indeed, the MEA is heavily dehumidified, in particular at the entry of the cathode gas into the cathode channel. This dehumidifying is caused by the water vaporization due to the high temperatures which occur in the electrochemical reaction and during current conveyance. This effect is reinforced by the electro-osmotic conveyance of hydrogen-water compounds, e.g., hydronium ions H (H2O), which convey water from the anode to the cathode.
Local desiccation of the MEA results in lower proton conductivity and thus in a reduction in the cell voltage and the efficiency of the fuel cell. In addition, due to the increasing amount of water produced in the remaining portion of the cathode channel, the dew point and the water vapor partial pressure of the cathode gas normally increase. As a consequence of the water vapor partial pressure increase, the oxygen partial pressure decreases along the cathode channel which also results in a reduction of the cell voltage and thus in a reduction of the efficiency.
U.S. Pat. No. 4,769,297 describes a fuel cell stack having porous electrodes and hydrophilic, porous distribution plates. The distribution plates absorb the water, produced on the cathode side during the electrochemical reaction, and convey it from the cathode side of the distribution plate to the anode side of an adjacent fuel cell, thus humidifying the MEA of the adjacent fuel cell. Simultaneously, the distribution plate conveys the water produced to the inlet of the cathode gas channel, thereby additionally humidifying the cathode gas.
The high manufacturing costs, as well as the increased plate thickness, are disadvantages of the hydrophilic, porous distribution plates. Further disadvantages are the high sensitivity of the distribution plates with regard to high cell temperatures, as well as the decrease in efficiency due to the additional oxygen conveyance, made possible by the porous distribution plate between adjacent fuel cells.
For regulating the humidity of the cathode gas, U.S. Pat. No. 4,973,530 proposes a fuel cell stack which uses a further medium, e.g., water, for regulating the humidity of the cathode gas, thus having an additional separate fluid circuit. The distribution plates of adjacent fuel cells have two adjacent flow-connected channel sections. In the first channel section, the cathode gas is guided onto the MEA. After flowing through the first channel section, the cathode gas flows into the second channel section and is guided there to a water-permeable membrane. On the other side of this water-permeable membrane, water is guided along, so that the cathode gas may be humidified in this second channel section. In a further embodiment, U.S. Pat. No. 4,973,530 also describes the simultaneous regulation of the humidity of the anode gas. A disadvantage is the fact that at least one additional medium and thus an additional fluid circuit are required in this system. This results in further disadvantages with regard to a compact design of the fuel cell stack. Compared to conventional fuel cell stacks, this concept also has disadvantages with regard to the reduced efficiency due to a reduced surface of the MEA.